Below this introduction is the FAQ file included with the free and open source “cryptsetup” package used to set up full disk or partition encryption on Linux computers. A bit long and involved but a good read. You may already have this file if you have Cryptsetup installed but never have seen it.

An interesting tidbit from it is this: The US Secret Service (those who will most likely try to decrypt a domestic opponent’s computer) use as their passphrase cracking cluster the office computers of all 110,000 employees of the US Treasury. These are mostly ordinary office PC’s, meaning they can compute one hashed LUKS key per second each, same as your computer did when you made the encrypted disk. That’s only 110,000 passphrases per second for the whole system targetted on any one machine. That’s exceptionally poor performance compared to the 25 graphics card (GPU) cluster that in a benchmark was able to compute 63 BILLION phassphrase hashes per second. It is also only a tenth as fast as the Secret Service claimed almost five years ago to the Washington Post. Remember, that’s for ALL computers with encryption judged important enough for the Secret Service to be asked to attempt to decrypt. In a banking fraud case, a year’s effort on their part failed to decrypt a Truecrypt volume and the banker would not say shit. Only waterboarding was going to decrypt that one, and the US government never waterboards crooked bankers.

This kind of computing is similar to Bitcoin mining (also a hash computation), so expect the future threat to come from bitcoin mining hardware. That’s right, a Neo-Nazi with a single bitcoin mining box may be more able to get into your encrypted computer than the entire US Secret Service! Assume someone at Secret Service also owns such a box, even if just at home, expect about 2 billion passphrases/second if is uses a single top of the line AMD Radeon GPU.

This is the FAQ (Frequently Asked Questions) for cryptsetup. It
covers Linux disk encryption with plain dm-crypt (one passphrase,
no management, no metadata on disk) and LUKS (multiple user keys
with one master key, anti-forensic features, metadata block at
start of device, …). The latest version of this FAQ should
usually be available athttp://code.google.com/p/cryptsetup/wiki/FrequentlyAskedQuestions

* 1.2 WARNINGS

ATTENTION: If you are going to read just one thing, make it the
section on Backup and Data Recovery. By far the most questions on
the cryptsetup mailing list are from people that managed to damage
the start of their LUKS partitions, i.e. the LUKS header. In
most cases, there is nothing that can be done to help these poor
souls recover their data. Make sure you understand the problem and
limitations imposed by the LUKS security model BEFORE you face
such a disaster! In particular, make sure you have a current header
backup before doing any potentially dangerous operations.

SSDs/FLASH DRIVES: SSDs and Flash are different. Currently it is
unclear how to get LUKS or plain dm-crypt to run on them with the
full set of security features intact. This may or may not be a
problem, depending on the attacher model. See Section 5.19.

BACKUP: Yes, encrypted disks die, just as normal ones do. A full
backup is mandatory, see Section “6. Backup and Data Recovery” on
options for doing encrypted backup.

CLONING/IMAGING: If you clone or image a LUKS container, you make a
copy of the LUKS header and the master key will stay the same!
That means that if you distribute an image to several machines, the
same master key will be used on all of them, regardless of whether
you change the passphrases. Do NOT do this! If you do, a root-user
on any of the machines with a mapped (decrypted) container or a
passphrase on that machine can decrypt all other copies, breaking
security. See also Item 6.15.

DISTRIBUTION INSTALLERS: Some distribution installers offer to
create LUKS containers in a way that can be mistaken as activation
of an existing container. Creating a new LUKS container on top of
an existing one leads to permanent, complete and irreversible data
loss. It is strongly recommended to only use distribution
installers after a complete backup of all LUKS containers has been
made.

UBUNTU INSTALLER: In particular the Ubuntu installer seems to be
quite willing to kill LUKS containers in several different ways.
Those responsible at Ubuntu seem not to care very much (it is very
easy to recognize a LUKS container), so treat the process of
installing Ubuntu as a severe hazard to any LUKS container you may
have.

NO WARNING ON NON-INTERACTIVE FORMAT: If you feed cryptsetup from
STDIN (e.g. via GnuPG) on LUKS format, it does not give you the
warning that you are about to format (and e.g. will lose any
pre-existing LUKS container on the target), as it assumes it is
used from a script. In this scenario, the responsibility for
warning the user and possibly checking for an existing LUKS header
is shifted to the script. This is a more general form of the
previous item.

LUKS PASSPHRASE IS NOT THE MASTER KEY: The LUKS passphrase is not
used in deriving the master key. It is used in decrypting a master
key that is randomly selected on header creation. This means that
if you create a new LUKS header on top of an old one with
exactly the same parameters and exactly the same passphrase as the
old one, it will still have a different master key and your data
will be permanently lost.

PASSPHRASE CHARACTER SET: Some people have had difficulties with
this when upgrading distributions. It is highly advisable to only
use the 95 printable characters from the first 128 characters of
the ASCII table, as they will always have the same binary
representation. Other characters may have different encoding
depending on system configuration and your passphrase will not
work with a different encoding. A table of the standardized first
128 ASCII characters can, e.g. be found onhttp://en.wikipedia.org/wiki/ASCII

* 1.3 System specific warnings

– Ubuntu as of 4/2011: It seems the installer offers to create
LUKS partitions in a way that several people mistook for an offer
to activate their existing LUKS partition. The installer gives no
or an inadequate warning and will destroy your old LUKS header,
causing permanent data loss. See also the section on Backup and
Data Recovery.

Update 4/2013: I am still unsure whether this has been fixed by
now, best be careful. They also seem to have added even more LUKS
killer functionality to the Ubuntu installer. I can only strongly
recommended to not install Ubuntu on a system with existing LUKS
containers without complete backups.

* 1.4 My LUKS-device is broken! Help!

First: Do not panic! In many cases the data is still recoverable.
Do not do anything hasty! Steps:

– Take some deep breaths. Maybe add some relaxing music. This may
sound funny, but I am completely serious. Often, critical damage is
done only after the initial problem.

– Do not reboot. The keys mays still be in the kernel if the device
is mapped.

– Make sure others do not reboot the system.

– Do not write to your disk without a clear understanding why this
will not make matters worse. Do a sector-level backup before any
writes. Often you do not need to write at all to get enough access
to make a backup of the data.

Other contributors are listed at the end. If you want to contribute,
send your article, including a descriptive headline, to the
maintainer, or the dm-crypt mailing list with something like “FAQ
…” in the subject. You can also send more raw information and
have me write the section. Please note that by contributing to this
FAQ, you accept the license described below.

This work is under the “Attribution-Share Alike 3.0 Unported”
license, which means distribution is unlimited, you may create
derived works, but attributions to original authors and this
license statement must be retained and the derived work must be
under the same license. Seehttp://creativecommons.org/licenses/by-sa/3.0/ for more details of
the license.

Side note: I did text license research some time ago and I think
this license is best suited for the purpose at hand and creates the
least problems.

Instructions on how to subscribe to the mailing-list are at on the
project website. People are generally helpful and friendly on the
list.

The question of how to unsubscribe from the list does crop up
sometimes. For this you need your list management URL, which is
sent to you initially and once at the start of each month. Go to
the URL mentioned in the email and select “unsubscribe”. This page
also allows you to request a password reminder.

Alternatively, you can send an Email to dm-crypt-request@saout.de
with just the word “help” in the subject or message body. Make sure
to send it from your list address.

Send mail to dm-crypt-unsubscribe@saout.de from the subscribed
account. You will get an email with instructions.

Basically, you just have to respond to it unmodified to get
unsubscribed. The listserver admin functions are not very fast. It
can take 15 minutes or longer for a reply to arrive (I suspect
greylisting is in use), so be patient.

Also note that nobody on the list can unsubscribe you, sending
demands to be unsubscribed to the list just annoys people that are
entirely blameless for you being subscribed.

If you are subscribed, a subscription confirmation email was sent
to your email account and it had to be answered before the
subscription went active. The confirmation emails from the
listserver have subjects like these (with other numbers):

Subject: confirm 9964cf10…..

and are sent from dm-crypt-request@saout.de. You should check
whether you have anything like it in your sent email folder. If
you find nothing and are sure you did not confirm, then you should
look into a possible compromise of your email account.

2. Setup

* 2.1 LUKS Container Setup mini-HOWTO

This item tries to give you a very brief list of all the steps you
should go though when creating a new LUKS encrypted container, i.e.
encrypted disk, partition or loop-file.

01) All data will be lost, if there is data on the target, make a
backup.

02) Make very sure you have the right target disk, partition or
loop-file.

03) If the target was in use previously, it is a good idea to
wipe it before creating the LUKS container in order to remove any
trace of old file systems and data. For example, some users have
managed to run e2fsck on a partition containing a LUKS container,
possibly because of residual ext2 superblocks from an earlier use.
This can do arbitrary damage up to complete and permanent loss of
all data in the LUKS container.

To just quickly wipe file systems (old data may remain), use

wipefs -a

To wipe file system and data, use something like

cat /dev/zero >

This can take a while. To get a progress indicator, you can use
the tool dd_rescue (->google) instead or use my stream meter “wcs”
(source here: http://www.tansi.org/tools/index.html) in the
following fashion:

cat /dev/zero | wcs >

Be very sure you have the right target, all data will be lost!

Note that automatic wiping is on the TODO list for cryptsetup, so
at some time in the future this will become unnecessary.

04) Create the LUKS container:
cryptsetup luksFormat

Just follow the on-screen instructions.

Note: Passphrase iteration is determined by cryptsetup depending on
CPU power. On a slow device, this may be lower than you want. I
recently benchmarked this on a Raspberry Pi and it came out at
about 1/15 of the iteration count for a typical PC. If security is
paramount, you may want to increase the time spent in iteration, at
the cost of a slower unlock later. For the Raspberry Pi, using

cryptsetup luksFormat -i 15000

gives you an iteration count and security level equal to an average
PC for passphrase iteration and master-key iteration. If in doubt,
check the iteration counts with

cryptsetup luksDump

and adjust the iteration count accordingly by creating the container
again with a different iteration time (the number after ‘-i’ is the
iteration time in milicesonds) until your requirements are met.

05) Map the container. Here it will be mapped to /dev/mapper/c1:
cryptsetup luksOpen c1

06) (Optionally) wipe the container (make sure you have the right target!):
cat /dev/zero > /dev/mapper/c1

Note that this creates a small information leak, as an attacker can
determine whether a 512 byte block is zero if the attacker has
access to the encrypted container multiple times. Typically a
competent attacker that has access multiple times can install a
passphrase sniffer anyways, so this leakage is not very
significant. For getting a progress indicator, see step 03.

Note that at some time in the future, cryptsetup will do this for
you, but currently it is a TODO list item.

07) Create a file system in the mapped container, for example an
ext3 file system (any other file system is possible):

Done. You can now use the encrypted file system to store data. Be
sure to read though the rest of the FAQ, these are just the very
basics. In particular, there are a number of mistakes that are
easy to make, but will compromise your security.

* 2.2 How do I set up encrypted swap?

As things that are confidential can end up in swap (keys,
passphrases, etc. are usually protected against being swapped to
disk, but other things may not be), it may be advisable to do
something about the issue. One option is to run without swap, which
generally works well in a desktop-context. It may cause problems
in a server-setting or under special circumstances. The solution to
that is to encrypt swap with a random key at boot-time.

NOTE: This is for Debian, and should work for Debian-derived
distributions. For others you may have to write your own startup
script or use other mechanisms.

01) Add the swap partition to /etc/crypttab. A line like the following
should do it:

swap /dev/ /dev/urandom swap,noearly

Warning: While Debian refuses to overwrite partitions with a
filesystem or RAID signature on it, if your disk IDs may change
(adding or removing disks, failure of disk during boot, etc.), you
may want to take additional precautions. Yes, this means that your
kernel device names like sda, sdb, … can change between reboots!
This is not a concern if you have only one disk. One possibility is
to make sure the partition number is not present on additional
disks or also swap there. Another is to encapsulate the swap
partition (by making it a 1-disk RAID1 or by using LVM), so that it
gets a persistent identifier. Specifying it directly by UUID does
not work, unfortunately, as the UUID is part of the swap signature
and that is not visible from the outside due to the encryption and
in addition changes on each reboot with this setup.

Note: Use /dev/random if you are paranoid or in a potential
low-entropy situation (embedded system, etc.). This may cause the
operation to take a long time during boot. If you are in a “no
entropy” situation, you cannot encrypt swap securely. In this
situation you should find some entropy, also because nothing else
using crypto will be secure, like ssh, ssl or GnuPG.

Note: The “noearly” option makes sure things like LVM, RAID, etc.
are running. As swap is non-critical for boot, it is fine to start
it late.

02) Add the swap partition to /etc/fstab. A line like the following
should do it:

/dev/mapper/swap none swap sw 0 0

That is it. Reboot or start it manually to activate encrypted swap.
Manual start would look like this:

/etc/init.d/crypdisks start
swapon /dev/mapper/swap

* 2.3 What is the difference between “plain” and LUKS format?

First, unless you happen to understand the cryptographic background
well, you should use LUKS. It does protect the user from a lot of
common mistakes. Plain dm-crypt is for experts.

Plain format is just that: It has no metadata on disk, reads all
parameters from the commandline (or the defaults), derives a
master-key from the passphrase and then uses that to de-/encrypt
the sectors of the device, with a direct 1:1 mapping between
encrypted and decrypted sectors.

Primary advantage is high resilience to damage, as one damaged
encrypted sector results in exactly one damaged decrypted sector.
Also, it is not readily apparent that there even is encrypted data
on the device, as an overwrite with crypto-grade randomness (e.g.
from /dev/urandom) looks exactly the same on disk.

Side-note: That has limited value against the authorities. In
civilized countries, they cannot force you to give up a crypto-key
anyways. In quite a few countries around the world, they can force
you to give up the keys (using imprisonment or worse to pressure
you, sometimes without due process), and in the worst case, they
only need a nebulous “suspicion” about the presence of encrypted
data. Sometimes this applies to everybody, sometimes only when you
are suspected of having “illicit data” (definition subject to
change) and sometimes specifically when crossing a border. Note
that this is going on in countries like the US and the UK, to
different degrees and sometimes with courts restricting what the
authorities can actually demand.

My advice is to either be ready to give up the keys or to not have
encrypted data when traveling to those countries, especially when
crossing the borders. The latter also means not having any
high-entropy (random) data areas on your disk, unless you can
explain them and demonstrate that explanation. Hence doing a
zero-wipe of all free space, including unused space, may be a good
idea.

Disadvantages are that you do not have all the nice features that
the LUKS metadata offers, like multiple passphrases that can be
changed, the cipher being stored in the metadata, anti-forensic
properties like key-slot diffusion and salts, etc..

LUKS format uses a metadata header and 8 key-slot areas that are
being placed at the beginning of the disk, see below under “What
does the LUKS on-disk format looks like?”. The passphrases are used
to decrypt a single master key that is stored in the anti-forensic
stripes.

Advantages are a higher usability, automatic configuration of
non-default crypto parameters, defenses against low-entropy
passphrases like salting and iterated PBKDF2 passphrase hashing,
the ability to change passphrases, and others.

Disadvantages are that it is readily obvious there is encrypted
data on disk (but see side note above) and that damage to the
header or key-slots usually results in permanent data-loss. See
below under “6. Backup and Data Recovery” on how to reduce that
risk. Also the sector numbers get shifted by the length of the
header and key-slots and there is a loss of that size in capacity
(1MB+4096B for defaults and 2MB for the most commonly used
non-default XTS mode).

* 2.4 Can I encrypt an already existing, non-empty partition to use
LUKS?

There is no converter, and it is not really needed. The way to do
this is to make a backup of the device in question, securely wipe
the device (as LUKS device initialization does not clear away old
data), do a luksFormat, optionally overwrite the encrypted device,
create a new filesystem and restore your backup on the now
encrypted device. Also refer to sections “Security Aspects” and
“Backup and Data Recovery”.

For backup, plain GNU tar works well and backs up anything likely
to be in a filesystem.

* 2.5 How do I use LUKS with a loop-device?

This can be very handy for experiments. Setup is just the same as
with any block device. If you want, for example, to use a 100MiB
file as LUKS container, do something like this:

Afterwards just use /dev/loop0 as a you would use a LUKS partition.
To unmap the file when done, use “losetup -d /dev/loop0”.

* 2.6 When I add a new key-slot to LUKS, it asks for a passphrase but
then complains about there not being a key-slot with that
passphrase?

That is as intended. You are asked a passphrase of an existing
key-slot first, before you can enter the passphrase for the new
key-slot. Otherwise you could break the encryption by just adding a
new key-slot. This way, you have to know the passphrase of one of
the already configured key-slots in order to be able to configure a
new key-slot.

* 2.7 Encryption on top of RAID or the other way round?

Unless you have special needs, place encryption between RAID and
filesystem, i.e. encryption on top of RAID. You can do it the other
way round, but you have to be aware that you then need to give the
passphrase for each individual disk and RAID autodetection will
not work anymore. Therefore it is better to encrypt the RAID
device, e.g. /dev/dm0 .

* 2.12 If I map a journaled file system using dm-crypt/LUKS, does it
still provide its usual transactional guarantees?

Yes, it does, unless a very old kernel is used. The required flags
come from the filesystem layer and are processed and passed onwards
by dm-crypt. A bit more information on the process by which
transactional guarantees are implemented can be found here:

Please note that these “guarantees” are weaker than they appear to
be. One problem is that quite a few disks lie to the OS about
having flushed their buffers. Some other things can go wrong as
well. The filesystem developers are aware of these problems and
typically can make it work anyways. That said, dm-crypt/LUKS will
not make things worse.

One specific problem you can run into though is that you can get
short freezes and other slowdowns due to the encryption layer.
Encryption takes time and forced flushes will block for that time.
For example, I did run into frequent small freezes (1-2 sec) when
putting a vmware image on ext3 over dm-crypt. When I went back to
ext2, the problem went away. This seems to have gotten better with
kernel 2.6.36 and the reworking of filesystem flush locking
mechanism (less blocking of CPU activity during flushes). It
should improve further and eventually the problem should go away.

* 2.13 Can I use LUKS or cryptsetup with a more secure (external)
medium for key storage, e.g. TPM or a smartcard?

Yes, see the answers on using a file-supplied key. You do have to
write the glue-logic yourself though. Basically you can have
cryptsetup read the key from STDIN and write it there with your
own tool that in turn gets the key from the more secure key
storage.

Yes, you can, as neither dm-crypt nor LUKS stores partition size.
Whether you should is a different question. Personally I recommend
backup, recreation of the encrypted partition with new size,
recreation of the filesystem and restore. This gets around the
tricky business of resizing the filesystem. Resizing a dm-crypt or
LUKS container does not resize the filesystem in it. The backup is
really non-optional here, as a lot can go wrong, resulting in
partial or complete data loss. Using something like gparted to
resize an encrypted partition is slow, but typically works. This
will not change the size of the filesystem hidden under the
encryption though.

You also need to be aware of size-based limitations. The one
currently relevant is that aes-xts-plain should not be used for
encrypted container sizes larger than 2TiB. Use aes-xts-plain64
for that.

* 2.15 How do I Benchmark the Ciphers, Hashes and Modes?

Since version 1.60 cryptsetup supports the “benchmark” command.
Simply run as root:

cryptsetup benchmark

It will output first iterations/second for the key-derivation
function PBKDF2 parameterized with different hash-functions, and
then the raw encryption speed of ciphers with different modes and
key-sizes. You can get more than the default benchmarks, see the
man-page for the relevant parameters. Note that XTS mode takes two
keys, hence the listed key sizes are double that for other modes
and half of it is the cipher key, the other half is the XTS key.

* 2.16 How do I Verify I have an Authentic cryptsetup Source Package?

Current maintainer is Milan Broz and he signs the release packages
with his PGP key. The key he currently uses is the “RSA key ID
D93E98FC”, fingerprint 2A29 1824 3FDE 4664 8D06 86F9 D9B0 577B
D93E 98FC. While I have every confidence this really is his key and
that he is who he claims to be, don’t depend on it if your life is
at stake. For that matter, if your life is at stake, don’t depend
on me being who I claim to be either.

That said, as cryptsetup is under good version control, a malicious
change should be noticed sooner or later, but it may take a while.
Also, the attacker model makes compromising the sources in a
non-obvious way pretty hard. Sure, you could put the master-key
somewhere on disk, but that is rather obvious as soon as somebody
looks as there would be data in an empty LUKS container in a place
it should not be. Doing this in a more nefarious way, for example
hiding the master-key in the salts, would need a look at the
sources to be discovered, but I think that somebody would find that
sooner or later as well.

That said, this discussion is really a lot more complicated and
longer as an FAQ can sustain. If in doubt, ask on the mailing list.

* 2.17 Is there a concern with 4k Sectors?

Not from dm-crypt itself. Encryption will be done in 512B blocks,
but if the partition and filesystem are aligned correctly and the
filesystem uses multiples of 4kiB as block size, the dm-crypt layer
will just process 8 x 512B = 4096B at a time with negligible
overhead. LUKS does place data at an offset, which is 2MiB per
default and will not break alignment. See also Item 6.12 of this
FAQ for more details. Note that if your partition or filesystem is
misaligned, dm-crypt can make the effect worse though.

3. Common Problems

* 3.1 My dm-crypt/LUKS mapping does not work! What general steps are
there to investigate the problem?

If you get a specific error message, investigate what it claims
first. If not, you may want to check the following things.

– Check that “/dev”, including “/dev/mapper/control” is there. If it
is missing, you may have a problem with the “/dev” tree itself or
you may have broken udev rules.

– Check that you have the device mapper and the crypt target in your
kernel. The output of “dmsetup targets” should list a “crypt”
target. If it is not there or the command fails, add device mapper
and crypt-target to the kernel.

– Check that the hash-functions and ciphers you want to use are in
the kernel. The output of “cat /proc/crypto” needs to list them.

* 3.2 My dm-crypt mapping suddenly stopped when upgrading cryptsetup.

The default cipher, hash or mode may have changed (the mode changed
from 1.0.x to 1.1.x). See under “Issues With Specific Versions of
cryptsetup”.

* 3.3 When I call cryptsetup from cron/CGI, I get errors about
unknown features?

If you get errors about unknown parameters or the like that are not
present when cryptsetup is called from the shell, make sure you
have no older version of cryptsetup on your system that then gets
called by cron/CGI. For example some distributions install
cryptsetup into /usr/sbin, while a manual install could go to
/usr/local/sbin. As a debugging aid, call “cryptsetup –version”
from cron/CGI or the non-shell mechanism to be sure the right
version gets called.

* 3.4 Unlocking a LUKS device takes very long. Why?

The iteration time for a key-slot (see Section 5 for an explanation
what iteration does) is calculated when setting a passphrase. By
default it is 1 second on the machine where the passphrase is set.
If you set a passphrase on a fast machine and then unlock it on a
slow machine, the unlocking time can be much longer. Also take into
account that up to 8 key-slots have to be tried in order to find the
right one.

If this is problem, you can add another key-slot using the slow
machine with the same passphrase and then remove the old key-slot.
The new key-slot will have an iteration count adjusted to 1 second
on the slow machine. Use luksKeyAdd and then luksKillSlot or
luksRemoveKey.

However, this operation will not change volume key iteration count
(MK iterations in output of “cryptsetup luksDump”). In order to
change that, you will have to backup the data in the LUKS
container (i.e. your encrypted data), luksFormat on the slow
machine and restore the data. Note that in the original LUKS
specification this value was fixed to 10, but it is now derived
from the PBKDF2 benchmark as well and set to iterations in 0.125
sec or 1000, whichever is larger. Also note that MK iterations
are not very security relevant. But as each key-slot already takes
1 second, spending the additional 0.125 seconds really does not
matter.

* 3.5 “blkid” sees a LUKS UUID and an ext2/swap UUID on the same
device. What is wrong?

Some old versions of cryptsetup have a bug where the header does
not get completely wiped during LUKS format and an older ext2/swap
signature remains on the device. This confuses blkid.

Fix: Wipe the unused header areas by doing a backup and restore of
the header with cryptsetup 1.1.x:

There seems to be some interference between the hardening and and
the way cryptsetup benchmarks PBKDF2. The solution to this is
currently not quite clear for an encrypted root filesystem. For
other uses, you can apparently specify USE=”dynamic” as compile
flag, see http://bugs.gentoo.org/show_bug.cgi?id=283470

4. Troubleshooting

* 4.1 I get the error “LUKS keyslot x is invalid.” What does that
mean?

This means that the given keyslot has an offset that points
outside the valid keyslot area. Typically, the reason is a
corrupted LUKS header because something was written to the start of
the device the LUKS container is on. Refer to Section “Backup and
Data Recovery” and ask on the mailing list if you have trouble
diagnosing and (if still possible) repairing this.

* 4.2 I cannot unlock my LUKS container! What could be the problem?

First, make sure you have a correct passphrase. Then make sure you
have the correct key-map and correct keyboard. And then make sure
you have the correct character set and encoding, see also
“PASSPHRASE CHARACTER SET” under Section 1.2.

If you are sure you are entering the passphrase right, there is the
possibility that the respective key-slot has been damaged. There
is no way to recover a damaged key-slot, except from a header
backup (see Section 6). For security reasons, there is also no
checksum in the key-slots that could tell you whether a key-slot has
been damaged. The only checksum present allows recognition of a
correct passphrase, but that only works if the passphrase is
correct and the respective key-slot is intact.

In order to find out whether a key-slot is damaged one has to look
for “non-random looking” data in it. There is a tool that
automatizes this in the cryptsetup distribution from version 1.6.0
onwards. It is located in misc/keyslot_checker/. Instructions how
to use and how to interpret results are in the README file. Note
that this tool requires a libcryptsetup from cryptsetup 1.6.0 or
later (which means libcryptsetup.so.4.5.0 or later). If the tool
complains about missing functions in libcryptsetup, you likely
have an earlier version from your distribution still installed. You
can either point the symbolic link(s) from libcryptsetup.so.4 to
the new version manually, or you can uninstall the distribution
version of cryptsetup and re-install that from cryptsetup >= 1.6.0
again to fix this.

* 4.3 Can a bad RAM module cause problems?

LUKS and dm-crypt can give the RAM quite a workout, especially when
combined with software RAID. In particular the combination RAID5 +
LUKS + XFS seems to uncover RAM problems that never caused obvious
problems before. Symptoms vary, but often the problem manifest
itself when copying large amounts of data, typically several times
larger than your main memory.

Side note: One thing you should always do on large data
copy/movements is to run a verify, for example with the “-d”
option of “tar” or by doing a set of MD5 checksums on the source
or target with

find . -type f -exec md5sum \{\} \; > checksum-file

and then a “md5sum -c checksum-file” on the other side. If you get
mismatches here, RAM is the primary suspect. A lesser suspect is
an overclocked CPU. I have found countless hardware problems in
verify runs after copying or making backups. Bit errors are much
more common than most people think.

Some RAM issues are even worse and corrupt structures in one of the
layers. This typically results in lockups, CPU state dumps in the
system logs, kernel panic or other things. It is quite possible to
have the problem with an encrypted device, but not with an
otherwise the same unencrypted device. The reason for that is that
encryption has an error amplification property: You flip one bit
in an encrypted data block, and the decrypted version has half of
its bits flipped. This is an important security property for modern
ciphers. With the usual modes in cryptsetup (CBC, ESSIV, XTS), you
get up to a completely changed 512 byte block per bit error. A
corrupt block causes a lot more havoc than the occasionally
flipped single bit and can result in various obscure errors.

Note, that a verify run on copying between encrypted or
unencrypted devices will reliably detect corruption, even when the
copying itself did not report any problems. If you find defect
RAM, assume all backups and copied data to be suspect, unless you
did a verify.

* 4.4 How do I test RAM?

First you should know that overclocking often makes memory
problems worse. So if you overclock (which I strongly recommend
against in a system holding data that has some worth), run the
tests with the overclocking active.

There are two good options. One is Memtest86+ and the other is
“memtester” by Charles Cazabon. Memtest86+ requires a reboot and
then takes over the machine, while memtester runs from a
root-shell. Both use different testing methods and I have found
problems fast with each one that the other needed long to find. I
recommend running the following procedure until the first error is
found:

– Run Memtest86+ for one cycle

– Run memtester for one cycle (shut down as many other applications
as possible)

– Run Memtest86+ for 24h or more

– Run memtester for 24h or more

If all that does not produce error messages, your RAM may be sound,
but I have had one weak bit that Memtest86+ needed around 60 hours
to find. If you can reproduce the original problem reliably, a good
additional test may be to remove half of the RAM (if you have more
than one module) and try whether the problem is still there and if
so, try with the other half. If you just have one module, get a
different one and try with that. If you do overclocking, reduce
the settings to the most conservative ones available and try with
that.

5. Security Aspects

* 5.1 How long is a secure passphrase ?

This is just the short answer. For more info and explanation of
some of the terms used in this item, read the rest of Section 5.
The actual recommendation is at the end of this item.

First, passphrase length is not really the right measure,
passphrase entropy is. For example, a random lowercase letter (a-z)
gives you 4.7 bit of entropy, one element of a-z0-9 gives you 5.2
bits of entropy, an element of a-zA-Z0-9 gives you 5.9 bits and
a-zA-Z0-9!@#$%^&:-+ gives you 6.2 bits. On the other hand, a random
English word only gives you 0.6…1.3 bits of entropy per
character. Using sentences that make sense gives lower entropy,
series of random words gives higher entropy. Do not use sentences
that can be tied to you or found on your computer. This type of
attack is done routinely today.

That said, it does not matter too much what scheme you use, but it
does matter how much entropy your passphrase contains, because an
attacker has to try on average

1/2 * 2^(bits of entropy in passphrase)

different passphrases to guess correctly.

Historically, estimations tended to use computing time estimates,
but more modern approaches try to estimate cost of guessing a
passphrase.

LUKS uses SHA1 for hashing per default. The claim in the reference
is 63 billion tries/second for SHA1. We will leave aside the check
whether a try actually decrypts a key-slot. Now, the machine has 25
GPUs, which I will estimate at an overall lifetime cost of USD/EUR
1000 each, and an useful lifetime of 2 years. (This is on the low
side.) Disregarding downtime, the machine can then break

N = 63*10^9 * 3600 * 24 * 365 * 2 ~ 4*10^18

passphrases for EUR/USD 25k. That is one 62 bit passphrase hashed
once with SHA1 for EUR/USD 25k. Note that as this can be
parallelized, it can be done faster than 2 years with several of
these machines.

For plain dm-crypt (no hash iteration) this is it. This gives (with
SHA1, plain dm-crypt default is ripemd160 which seems to be
slightly slower than SHA1):

To get reasonable security for the next 10 years, it is a good idea
to overestimate by a factor of at least 1000.

Then there is the question of how much the attacker is willing to
spend. That is up to your own security evaluation. For general use,
I will assume the attacker is willing to spend up to 1 million
EUR/USD. Then we get the following recommendations:

Plain dm-crypt: Use > 80 bit. That is e.g. 17 random chars from a-z
or a random English sentence of > 135 characters length.

LUKS: Use > 65 bit. That is e.g. 14 random chars from a-z or a
random English sentence of > 108 characters length.

If paranoid, add at least 20 bit. That is roughly four additional
characters for random passphrases and roughly 32 characters for a
random English sentence.

* 5.2 Is LUKS insecure? Everybody can see I have encrypted data!

In practice it does not really matter. In most civilized countries
you can just refuse to hand over the keys, no harm done. In some
countries they can force you to hand over the keys, if they suspect
encryption. However the suspicion is enough, they do not have to
prove anything. This is for practical reasons, as even the presence
of a header (like the LUKS header) is not enough to prove that you
have any keys. It might have been an experiment, for example. Or it
was used as encrypted swap with a key from /dev/random. So they
make you prove you do not have encrypted data. Of course that is
just as impossible as the other way round.

This means that if you have a large set of random-looking data,
they can already lock you up. Hidden containers (encryption hidden
within encryption), as possible with Truecrypt, do not help
either. They will just assume the hidden container is there and
unless you hand over the key, you will stay locked up. Don’t have
a hidden container? Though luck. Anybody could claim that.

Still, if you are concerned about the LUKS header, use plain
dm-crypt with a good passphrase. See also Section 2, “What is the
difference between “plain” and LUKS format?”

* 5.3 Should I initialize (overwrite) a new LUKS/dm-crypt partition?

If you just create a filesystem on it, most of the old data will
still be there. If the old data is sensitive, you should overwrite
it before encrypting. In any case, not initializing will leave the
old data there until the specific sector gets written. That may
enable an attacker to determine how much and where on the
partition data was written. If you think this is a risk, you can
prevent this by overwriting the encrypted device (here assumed to
be named “e1”) with zeros like this:

dd_rescue -w /dev/zero /dev/mapper/e1

or alternatively with one of the following more standard commands:

cat /dev/zero > /dev/mapper/e1
dd if=/dev/zero of=/dev/mapper/e1

* 5.4 How do I securely erase a LUKS (or other) partition?

For LUKS, if you are in a desperate hurry, overwrite the LUKS
header and key-slot area. This means overwriting the first
(keyslots x stripes x keysize) + offset bytes. For the default
parameters, this is the 1’052’672 bytes, i.e. 1MiB + 4096 of the
LUKS partition. For 512 bit key length (e.g. for aes-xts-plain with
512 bit key) this is 2MiB. (The different offset stems from
differences in the sector alignment of the key-slots.) If in doubt,
just be generous and overwrite the first 10MB or so, it will likely
still be fast enough. A single overwrite with zeros should be
enough. If you anticipate being in a desperate hurry, prepare the
command beforehand. Example with /dev/sde1 as the LUKS partition
and default parameters:

head -c 1052672 /dev/zero > /dev/sde1; sync

A LUKS header backup or full backup will still grant access to
most or all data, so make sure that an attacker does not have
access to backups or destroy them as well.

If you have time, overwrite the whole LUKS partition with a single
pass of zeros. This is enough for current HDDs. For SSDs or FLASH
(USB sticks) you may want to overwrite the whole drive several
times to be sure data is not retained by wear leveling. This is
possibly still insecure as SSD technology is not fully understood
in this regard. Still, due to the anti-forensic properties of the
LUKS key-slots, a single overwrite of an SSD or FLASH drive could
be enough. If in doubt, use physical destruction in addition. Here
is a link to some current research results on erasing SSDs and
FLASH drives:http://www.usenix.org/events/fast11/tech/full_papers/Wei.pdf

Keep in mind to also erase all backups.

Example for a zero-overwrite erase of partition sde1 done with
dd_rescue:

dd_rescue -w /dev/zero /dev/sde1

* 5.5 How do I securely erase a backup of a LUKS partition or header?

That depends on the medium it is stored on. For HDD and SSD, use
overwrite with zeros. For an SSD or FLASH drive (USB stick), you
may want to overwrite the complete SSD several times and use
physical destruction in addition, see last item. For re-writable
CD/DVD, a single overwrite should also be enough, due to the
anti-forensic properties of the LUKS keyslots. For write-once
media, use physical destruction. For low security requirements,
just cut the CD/DVD into several parts. For high security needs,
shred or burn the medium. If your backup is on magnetic tape, I
advise physical destruction by shredding or burning, after
overwriting . The problem with magnetic tape is that it has a
higher dynamic range than HDDs and older data may well be
recoverable after overwrites. Also write-head alignment issues can
lead to data not actually being deleted at all during overwrites.

* 5.6 What about backup? Does it compromise security?

That depends. See item 6.7.

* 5.7 Why is all my data permanently gone if I overwrite the LUKS
header?

Overwriting the LUKS header in part or in full is the most common
reason why access to LUKS containers is lost permanently.
Overwriting can be done in a number of fashions, like creating a
new filesystem on the raw LUKS partition, making the raw partition
part of a raid array and just writing to the raw partition.

The LUKS header contains a 256 bit “salt” value and without that no
decryption is possible. While the salt is not secret, it is
key-grade material and cannot be reconstructed. This is a
cryptographically strong “cannot”. From observations on the
cryptsetup mailing-list, people typically go though the usual
stages of grief (Denial, Anger, Bargaining, Depression, Acceptance)
when this happens to them. Observed times vary between 1 day and 2
weeks to complete the cycle. Seeking help on the mailing-list is
fine. Even if we usually cannot help with getting back your data,
most people found the feedback comforting.

If your header does not contain an intact salt, best go directly
to the last stage (“Acceptance”) and think about what to do now.
There is one exception that I know of: If your LUKS container is
still open, then it may be possible to extract the master key from
the running system. See Item “How do I recover the master key from
a mapped LUKS container?” in Section “Backup and Data Recovery”.

* 5.8 What is a “salt”?

A salt is a random key-grade value added to the passphrase before
it is processed. It is not kept secret. The reason for using salts
is as follows: If an attacker wants to crack the password for a
single LUKS container, then every possible passphrase has to be
tried. Typically an attacker will not try every binary value, but
will try words and sentences from a dictionary.

If an attacker wants to attack several LUKS containers with the
same dictionary, then a different approach makes sense: Compute the
resulting slot-key for each dictionary element and store it on
disk. Then the test for each entry is just the slow unlocking with
the slot key (say 0.00001 sec) instead of calculating the slot-key
first (1 sec). For a single attack, this does not help. But if you
have more than one container to attack, this helps tremendously,
also because you can prepare your table before you even have the
container to attack! The calculation is also very simple to
parallelize. You could, for example, use the night-time unused CPU
power of your desktop PCs for this.

This is where the salt comes in. If the salt is combined with the
passphrase (in the simplest form, just appended to it), you
suddenly need a separate table for each salt value. With a
reasonably-sized salt value (256 bit, e.g.) this is quite
infeasible.

* 5.9 Is LUKS secure with a low-entropy (bad) passphrase?

Note: You should only use the 94 printable characters from 7 bit
ASCII code to prevent your passphrase from failing when the
character encoding changes, e.g. because of a system upgrade, see
also the note at the very start of this FAQ under “WARNINGS”.

This needs a bit of theory. The quality of your passphrase is
directly related to its entropy (information theoretic, not
thermodynamic). The entropy says how many bits of “uncertainty” or
“randomness” are in you passphrase. In other words, that is how
difficult guessing the passphrase is.

Example: A random English sentence has about 1 bit of entropy per
character. A random lowercase (or uppercase) character has about
4.7 bit of entropy.

Now, if n is the number of bits of entropy in your passphrase and t
is the time it takes to process a passphrase in order to open the
LUKS container, then an attacker has to spend at maximum

attack_time_max = 2^n * t

time for a successful attack and on average half that. There is no
way getting around that relationship. However, there is one thing
that does help, namely increasing t, the time it takes to use a
passphrase, see next FAQ item.

Still, if you want good security, a high-entropy passphrase is the
only option. For example, a low-entropy passphrase can never be
considered secure against a TLA-level (Three Letter Agency level,
i.e. government-level) attacker, no matter what tricks are used in
the key-derivation function. Use at least 64 bits for secret stuff.
That is 64 characters of English text (but only if randomly chosen)
or a combination of 12 truly random letters and digits.

For passphrase generation, do not use lines from very well-known
texts (religious texts, Harry potter, etc.) as they are to easy to
guess. For example, the total Harry Potter has about 1’500’000
words (my estimation). Trying every 64 character sequence starting
and ending at a word boundary would take only something like 20
days on a single CPU and is entirely feasible. To put that into
perspective, using a number of Amazon EC2 High-CPU Extra Large
instances (each gives about 8 real cores), this test costs
currently about 50USD/EUR, but can be made to run arbitrarily fast.

On the other hand, choosing 1.5 lines from, say, the Wheel of Time
is in itself not more secure, but the book selection adds quite a
bit of entropy. (Now that I have mentioned it here, don’t use tWoT
either!) If you add 2 or 3 typos or switch some words around, then
this is good passphrase material.

* 5.10 What is “iteration count” and why is decreasing it a bad idea?

Iteration count is the number of PBKDF2 iterations a passphrase is
put through before it is used to unlock a key-slot. Iterations are
done with the explicit purpose to increase the time that it takes
to unlock a key-slot. This provides some protection against use of
low-entropy passphrases.

The idea is that an attacker has to try all possible passphrases.
Even if the attacker knows the passphrase is low-entropy (see last
item), it is possible to make each individual try take longer. The
way to do this is to repeatedly hash the passphrase for a certain
time. The attacker then has to spend the same time (given the same
computing power) as the user per try. With LUKS, the default is 1
second of PBKDF2 hashing.

Example 1: Lets assume we have a really bad passphrase (e.g. a
girlfriends name) with 10 bits of entropy. With the same CPU, an
attacker would need to spend around 500 seconds on average to
break that passphrase. Without iteration, it would be more like
0.0001 seconds on a modern CPU.

Example 2: The user did a bit better and has 32 chars of English
text. That would be about 32 bits of entropy. With 1 second
iteration, that means an attacker on the same CPU needs around 136
years. That is pretty impressive for such a weak passphrase.
Without the iterations, it would be more like 50 days on a modern
CPU, and possibly far less.

In addition, the attacker can both parallelize and use special
hardware like GPUs or FPGAs to speed up the attack. The attack can
also happen quite some time after the luksFormat operation and CPUs
can have become faster and cheaper. For that reason you want a
bit of extra security. Anyways, in Example 1 your are screwed.
In example 2, not necessarily. Even if the attack is faster, it
still has a certain cost associated with it, say 10000 EUR/USD
with iteration and 1 EUR/USD without iteration. The first can be
prohibitively expensive, while the second is something you try
even without solid proof that the decryption will yield something
useful.

The numbers above are mostly made up, but show the idea. Of course
the best thing is to have a high-entropy passphrase.

Would a 100 sec iteration time be even better? Yes and no.
Cryptographically it would be a lot better, namely 100 times better.
However, usability is a very important factor for security
technology and one that gets overlooked surprisingly often. For
LUKS, if you have to wait 2 minutes to unlock the LUKS container,
most people will not bother and use less secure storage instead. It
is better to have less protection against low-entropy passphrases
and people actually use LUKS, than having them do without
encryption altogether.

Now, what about decreasing the iteration time? This is generally a
very bad idea, unless you know and can enforce that the users only
use high-entropy passphrases. If you decrease the iteration time
without ensuring that, then you put your users at increased risk,
and considering how rarely LUKS containers are unlocked in a
typical work-flow, you do so without a good reason. Don’t do it.
The iteration time is already low enough that users with entropy
low passphrases are vulnerable. Lowering it even further increases
this danger significantly.

* 5.11 Some people say PBKDF2 is insecure?

There is some discussion that a hash-function should have a “large
memory” property, i.e. that it should require a lot of memory to be
computed. This serves to prevent attacks using special programmable
circuits, like FPGAs, and attacks using graphics cards. PBKDF2
does not need a lot of memory and is vulnerable to these attacks.
However, the publication usually referred in these discussions is
not very convincing in proving that the presented hash really is
“large memory” (that may change, email the FAQ maintainer when it
does) and it is of limited usefulness anyways. Attackers that use
clusters of normal PCs will not be affected at all by a “large
memory” property. For example the US Secret Service is known to
use the off-hour time of all the office PCs of the Treasury for
password breaking. The Treasury has about 110’000 employees.
Assuming every one has an office PC, that is significant computing
power, all of it with plenty of memory for computing “large
memory” hashes. Bot-net operators also have all the memory they
want. The only protection against a resourceful attacker is a
high-entropy passphrase, see items 5.9 and 5.10.

* 5.12 What about iteration count with plain dm-crypt?

Simple: There is none. There is also no salting. If you use plain
dm-crypt, the only way to be secure is to use a high entropy
passphrase. If in doubt, use LUKS instead.

* 5.13 Is LUKS with default parameters less secure on a slow CPU?

Unfortunately, yes. However the only aspect affected is the
protection for low-entropy passphrase or master-key. All other
security aspects are independent of CPU speed.

The master key is less critical, as you really have to work at it
to give it low entropy. One possibility is to supply the master key
yourself. If that key is low-entropy, then you get what you
deserve. The other known possibility is to use /dev/urandom for
key generation in an entropy-starved situation (e.g. automatic
installation on an embedded device without network and other entropy
sources).

For the passphrase, don’t use a low-entropy passphrase. If your
passphrase is good, then a slow CPU will not matter. If you insist
on a low-entropy passphrase on a slow CPU, use something like
“–iter-time=10” or higher and wait a long time on each LUKS unlock
and pray that the attacker does not find out in which way exactly
your passphrase is low entropy. This also applies to low-entropy
passphrases on fast CPUs. Technology can do only so much to
compensate for problems in front of the keyboard.

* 5.14 Why was the default aes-cbc-plain replaced with aes-cbc-essiv?

Note: This item applies both to plain dm-crypt and to LUKS

The problem is that cbc-plain has a fingerprint vulnerability, where
a specially crafted file placed into the crypto-container can be
recognized from the outside. The issue here is that for cbc-plain
the initialization vector (IV) is the sector number. The IV gets
XORed to the first data chunk of the sector to be encrypted. If you
make sure that the first data block to be stored in a sector
contains the sector number as well, the first data block to be
encrypted is all zeros and always encrypted to the same ciphertext.
This also works if the first data chunk just has a constant XOR
with the sector number. By having several shifted patterns you can
take care of the case of a non-power-of-two start sector number of
the file.

This mechanism allows you to create a pattern of sectors that have
the same first ciphertext block and signal one bit per sector to the
outside, allowing you to e.g. mark media files that way for
recognition without decryption. For large files this is a
practical attack. For small ones, you do not have enough blocks to
signal and take care of different file starting offsets.

In order to prevent this attack, the default was changed to
cbc-essiv. ESSIV uses a keyed hash of the sector number, with the
encryption key as key. This makes the IV unpredictable without
knowing the encryption key and the watermarking attack fails.

* 5.15 Are there any problems with “plain” IV? What is “plain64”?

First, “plain” and “plain64” are both not secure to use with CBC,
see previous FAQ item.

However there are modes, like XTS, that are secure with “plain” IV.
The next limit is that “plain” is 64 bit, with the upper 32 bit set
to zero. This means that on volumes larger than 2TiB, the IV
repeats, creating a vulnerability that potentially leaks some
data. To avoid this, use “plain64”, which uses the full sector
number up to 64 bit. Note that “plain64” requires a kernel >=
2.6.33. Also note that “plain64” is backwards compatible for
volume sizes 2TiB. Finally, “plain64”
does not cause any performance penalty compared to “plain”.

* 5.16 What about XTS mode?

XTS mode is potentially even more secure than cbc-essiv (but only if
cbc-essiv is insecure in your scenario). It is a NIST standard and
used, e.g. in Truecrypt. From version 1.6.0 of cryptsetup onwards,
aes-xts-plain64 is the default for LUKS. If you want to use it
with a cryptsetup before version 1.6.0 or with plain dm-crypt, you
have to specify it manually as “aes-xts-plain”, i.e.

cryptsetup -c aes-xts-plain luksFormat

For volumes >2TiB and kernels >= 2.6.33 use “plain64” (see FAQ
item on “plain” and “plain64”):

cryptsetup -c aes-xts-plain64 luksFormat

There is a potential security issue with XTS mode and large blocks.
LUKS and dm-crypt always use 512B blocks and the issue does not
apply.

* 5.17 Is LUKS FIPS-140-2 certified?

No. But that is more a problem of FIPS-140-2 than of LUKS. From a
technical point-of-view, LUKS with the right parameters would be
FIPS-140-2 compliant, but in order to make it certified, somebody
has to pay real money for that. And then, whenever cryptsetup is
changed or extended, the certification lapses and has to be
obtained again.

From the aspect of actual security, LUKS with default parameters
should be as good as most things that are FIPS-140-2 certified,
although you may want to make sure to use /dev/random (by
specifying –use-random on luksFormat) as randomness source for
the master key to avoid being potentially insecure in an
entropy-starved situation.

* 5.18 What about Plausible Deniability?

First let me attempt a definition for the case of encrypted
filesystems: Plausible deniability is when you hide encrypted data
inside an encrypted container and it is not possible to prove it is
there. The idea is compelling and on first glance it seems
possible to do it. And from a cryptographic point of view, it
actually is possible.

So, does it work in practice? No, unfortunately. The reasoning used
by its proponents is fundamentally flawed in several ways and the
cryptographic properties fail fatally when colliding with the real
world.

First, why should “I do not have a hidden partition” be any more
plausible than “I forgot my crypto key” or “I wiped that partition
with random data, nothing in there”? I do not see any reason.

Second, there are two types of situations: Either they cannot force
you to give them the key (then you simply do not) or the can. In
the second case, they can always do bad things to you, because they
cannot prove that you have the key in the first place! This means
they do not have to prove you have the key, or that this random
looking data on your disk is actually encrypted data. So the
situation will allow them to waterboard/lock-up/deport you
anyways, regardless of how “plausible” your deniability is. Do not
have a hidden partition you could show to them, but there are
indications you may? Too bad for you. Unfortunately “plausible
deniability” also means you cannot prove there is no hidden data.

Third, hidden partitions are not that hidden. There are basically
just two possibilities: a) Make a large crypto container, but put a
smaller filesystem in there and put the hidden partition into the
free space. Unfortunately this is glaringly obvious and can be
detected in an automated fashion. This means that the initial
suspicion to put you under duress in order to make you reveal you
hidden data is given. b) Make a filesystem that spans the whole
encrypted partition, and put the hidden partition into space not
currently used by that filesystem. Unfortunately that is also
glaringly obvious, as you then cannot write to the filesystem
without a high risk of destroying data in the hidden container.
Have not written anything to the encrypted filesystem in a while?
Too bad, they have the suspicion they need to do unpleasant things
to you.

To be fair, if you prepare option b) carefully and directly before
going into danger, it may work. But then, the mere presence of
encrypted data may already be enough to get you into trouble in
those places were they can demand encryption keys.

So, no, I will not provide any instructions on how to do it with
plain dm-crypt or LUKS. If you insist on shooting yourself in the
foot, you can figure out how to do it yourself.

* 5.19 What about SSDs, Flash and Hybrid Drives?

The problem is that you cannot reliably erase parts of these
devices, mainly due to wear-leveling and possibly defect
management.

Basically, when overwriting a sector (of 512B), what the device
does is to move an internal sector (may be 128kB or even larger) to
some pool of discarded, not-yet erased unused sectors, take a
fresh empty sector from the empty-sector pool and copy the old
sector over with the changes to the small part you wrote. This is
done in some fashion so that larger writes do not cause a lot of
small internal updates.

The thing is that the mappings between outside-addressable sectors
and inside sectors is arbitrary (and the vendors are not talking).
Also the discarded sectors are not necessarily erased immediately.
They may linger a long time.

For plain dm-crypt, the consequences are that older encrypted data
may be lying around in some internal pools of the device. Thus may
or may not be a problem and depends on the application. Remember
the same can happen with a filesystem if consecutive writes to the
same area of a file can go to different sectors.

However, for LUKS, the worst case is that key-slots and LUKS
header may end up in these internal pools. This means that password
management functionality is compromised (the old passwords may
still be around, potentially for a very long time) and that fast
erase by overwriting the header and key-slot area is insecure.

Also keep in mind that the discarded/used pool may be large. For
example, a 240GB SSD has about 16GB of spare area in the chips that
it is free to do with as it likes. You would need to make each
individual key-slot larger than that to allow reliable overwriting.
And that assumes the disk thinks all other space is in use.
Reading the internal pools using forensic tools is not that hard,
but may involve some soldering.

What to do?

If you trust the device vendor (you probably should not…) you can
try an ATA “secure erase” command for SSDs. That does not work for
USB keys though and may or may not be secure for a hybrid drive. If
it finishes on an SSD after a few seconds, it was possibly faked.
Unfortunately, for hybrid drives that indicator does not work, as
the drive may well take the time to truly erase the magnetic part,
but only mark the SSD/Flash part as erased while data is still in
there.

If you can do without password management and are fine with doing
physical destruction for permanently deleting data (always after
one or several full overwrites!), you can use plain dm-crypt or
LUKS.

If you want or need all the original LUKS security features to work,
you can use a detached LUKS header and put that on a conventional,
magnetic disk. That leaves potentially old encrypted data in the
pools on the disk, but otherwise you get LUKS with the same
security as on a magnetic disk.

If you are concerned about your laptop being stolen, you are likely
fine using LUKS on an SSD or hybrid drive. An attacker would need
to have access to an old passphrase (and the key-slot for this old
passphrase would actually need to still be somewhere in the SSD)
for your data to be at risk. So unless you pasted your old
passphrase all over the Internet or the attacker has knowledge of
it from some other source and does a targeted laptop theft to get
at your data, you should be fine.

* 5.20 LUKS is broken! It uses SHA-1!

No, it is not. SHA-1 is (academically) broken for finding
collisions, but not for using it in a key-derivation function. And
that collision vulnerability is for non-iterated use only. And you
need the hash-value in verbatim.

This basically means that if you already have a slot-key, and you
have set the PBKDF2 iteration count to 1 (it is > 10’000 normally),
you could (maybe) derive a different passphrase that gives you the
the same slot-key. But if you have the slot-key, you can already
unlock the key-slot and get the master key, breaking everything. So
basically, this SHA-1 vulnerability allows you to open a LUKS
container with high effort when you already have it open.

The real problem here is people that do not understand crypto and
claim things are broken just because some mechanism is used that
has been broken for a specific different use. The way the mechanism
is used matters very much. A hash that is broken for one use can be
completely secure for other uses and here it is.

* 5.21 Why is there no “Nuke-Option”?

A “Nuke-Option” or “Kill-switch” is a password that when entered
upon unlocking instead wipes the header and all passwords. So when
somebody forces you to enter your password, you can destroy the
data instead.

While this sounds attractive at first glance, it does not make sense
once a real security analysis is done. One problem is that you have
to have some kind of HSM (Hardware Security Module) in order to
implement it securely. In the movies, a HSM starts to smoke and
melt once the Nuke-Option has been activated. In reality, it just
wipes some battery-backed RAM cells. A proper HSM costs something
like 20’000…100’000 EUR/USD and there a Nuke-Option may make some
sense. BTW, a chipcard or a TPM is not a HSM, although some
vendors are promoting that myth.

Now, a proper HSMs will have a wipe option but not a Nuke-Option,
i.e. you can explicitly wipe the HSM, but by a different process
than unlocking it takes. Why is that? Simple: If somebody can force
you to reveal passwords, then they can also do bad things to you if
you do not or if you enter a nuke password instead. Think locking
you up for a few years for “destroying evidence” or for far longer
and without trial for being a “terrorist suspect”. No HSM maker
will want to expose its customers to that risk.

Now think of the typical LUKS application scenario, i.e. disk
encryption. Usually the ones forcing you to hand over your password
will have access to the disk as well, and, if they have any real
suspicion, they will mirror your disk before entering anything
supplied by you. This neatly negates any Nuke-Option. If they have
no suspicion (just harassing people that cross some border for
example), the Nuke-Option would work, but see above about likely
negative consequences and remember that a Nuke-Option may not work
reliably on SSD and hybrid drives anyways.

Hence my advice is to never take data that you do not want to reveal
into any such situation in the first place. There is no need to
transfer data on physical carriers today. The Internet makes it
quite possible to transfer data between arbitrary places and modern
encryption makes it secure. If you do it right, nobody will even be
able to identify source or destination. (How to do that is out of
scope of this document. It does require advanced skills in this age
of pervasive surveillance.)

Hence, LUKS has not kill option because it would do much more harm
than good.

Still, if you have a good use-case (i.e. non-abstract real-world
situation) where a Nuke-Option would actually be beneficial, please
let me know.

6. Backup and Data Recovery

* 6.1 Why do I need Backup?

First, disks die. The rate for well-treated (!) disk is about 5%
per year, which is high enough to worry about. There is some
indication that this may be even worse for some SSDs. This applies
both to LUKS and plain dm-crypt partitions.

Second, for LUKS, if anything damages the LUKS header or the
key-stripe area then decrypting the LUKS device can become
impossible. This is a frequent occurrence. For example an
accidental format as FAT or some software overwriting the first
sector where it suspects a partition boot sector typically makes a
LUKS partition permanently inaccessible. See more below on LUKS
header damage.

So, data-backup in some form is non-optional. For LUKS, you may
also want to store a header backup in some secure location. This
only needs an update if you change passphrases.

* 6.2 How do I backup a LUKS header?

While you could just copy the appropriate number of bytes from the
start of the LUKS partition, the best way is to use command option
“luksHeaderBackup” of cryptsetup. This protects also against
errors when non-standard parameters have been used in LUKS
partition creation. Example:

cryptsetup luksHeaderBackup –header-backup-file

To restore, use the inverse command, i.e.

cryptsetup luksHeaderRestore –header-backup-file

* 6.3 How do I test a LUKS header?

Use

cryptsetup -v isLuks

on the device. Without the “-v” it just signals its result via
exit-status. You can also use the more general test

blkid -p

which will also detect other types and give some more info. Omit
“-p” for old versions of blkid that do not support it.

* 6.4 How do I backup a LUKS or dm-crypt partition?

There are two options, a sector-image and a plain file or
filesystem backup of the contents of the partition. The sector
image is already encrypted, but cannot be compressed and contains
all empty space. The filesystem backup can be compressed, can
contain only part of the encrypted device, but needs to be
encrypted separately if so desired.

A sector-image will contain the whole partition in encrypted form,
for LUKS the LUKS header, the keys-slots and the data area. It can
be done under Linux e.g. with dd_rescue (for a direct image copy)
and with “cat” or “dd”. Example:

cat /dev/sda10 > sda10.img
dd_rescue /dev/sda10 sda10.img

You can also use any other backup software that is capable of making
a sector image of a partition. Note that compression is
ineffective for encrypted data, hence it does not make sense to
use it.

For a filesystem backup, you decrypt and mount the encrypted
partition and back it up as you would a normal filesystem. In this
case the backup is not encrypted, unless your encryption method
does that. For example you can encrypt a backup with “tar” as
follows with GnuPG:

tar cjf – | gpg –cipher-algo AES -c – > backup.tbz2.gpg

And verify the backup like this if you are at “path”:

cat backup.tbz2.gpg | gpg – | tar djf –

Note: Always verify backups, especially encrypted ones.

In both cases GnuPG will ask you interactively for your symmetric
key. The verify will only output errors. Use “tar dvjf -” to get
all comparison results. To make sure no data is written to disk
unencrypted, turn off swap if it is not encrypted before doing the
backup.

You can of course use different or no compression and you can use
an asymmetric key if you have one and have a backup of the secret
key that belongs to it.

A second option for a filesystem-level backup that can be used when
the backup is also on local disk (e.g. an external USB drive) is
to use a LUKS container there and copy the files to be backed up
between both mounted containers. Also see next item.

* 6.5 Do I need a backup of the full partition? Would the header and
key-slots not be enough?

Backup protects you against two things: Disk loss or corruption
and user error. By far the most questions on the dm-crypt mailing
list about how to recover a damaged LUKS partition are related
to user error. For example, if you create a new filesystem on a
LUKS partition, chances are good that all data is lost
permanently.

For this case, a header+key-slot backup would often be enough. But
keep in mind that a well-treated (!) HDD has roughly a failure
risk of 5% per year. It is highly advisable to have a complete
backup to protect against this case.

* *6.6 What do I need to backup if I use “decrypt_derived”?

This is a script in Debian, intended for mounting /tmp or swap with
a key derived from the master key of an already decrypted device.
If you use this for an device with data that should be persistent,
you need to make sure you either do not lose access to that master
key or have a backup of the data. If you derive from a LUKS
device, a header backup of that device would cover backing up the
master key. Keep in mind that this does not protect against disk
loss.

Note: If you recreate the LUKS header of the device you derive from
(using luksFormat), the master key changes even if you use the same
passphrase(s) and you will not be able to decrypt the derived
device with the new LUKS header.

* 6.7 Does a backup compromise security?

Depends on how you do it. However if you do not have one, you are
going to eventually lose your encrypted data.

There are risks introduced by backups. For example if you
change/disable a key-slot in LUKS, a binary backup of the partition
will still have the old key-slot. To deal with this, you have to
be able to change the key-slot on the backup as well, securely
erase the backup or do a filesystem-level backup instead of a binary
one.

If you use dm-crypt, backup is simpler: As there is no key
management, the main risk is that you cannot wipe the backup when
wiping the original. However wiping the original for dm-crypt
should consist of forgetting the passphrase and that you can do
without actual access to the backup.

In both cases, there is an additional (usually small) risk with
binary backups: An attacker can see how many sectors and which
ones have been changed since the backup. To prevent this, use a
filesystem level backup method that encrypts the whole backup in
one go, e.g. as described above with tar and GnuPG.

My personal advice is to use one USB disk (low value data) or
three disks (high value data) in rotating order for backups, and
either use independent LUKS partitions on them, or use encrypted
backup with tar and GnuPG.

If you do network-backup or tape-backup, I strongly recommend to
go the filesystem backup path with independent encryption, as you
typically cannot reliably delete data in these scenarios,
especially in a cloud setting. (Well, you can burn the tape if it
is under your control…)

* 6.8 What happens if I overwrite the start of a LUKS partition or
damage the LUKS header or key-slots?

There are two critical components for decryption: The salt values
in the header itself and the key-slots. If the salt values are
overwritten or changed, nothing (in the cryptographically strong
sense) can be done to access the data, unless there is a backup
of the LUKS header. If a key-slot is damaged, the data can still
be read with a different key-slot, if there is a remaining
undamaged and used key-slot. Note that in order to make a key-slot
unrecoverable in a cryptographically strong sense, changing about
4-6 bits in random locations of its 128kiB size is quite enough.

* 6.9 What happens if I (quick) format a LUKS partition?

I have not tried the different ways to do this, but very likely you
will have written a new boot-sector, which in turn overwrites the
LUKS header, including the salts, making your data permanently
irretrievable, unless you have a LUKS header backup. You may also
damage the key-slots in part or in full. See also last item.

* 6.10 How do I recover the master key from a mapped LUKS container?

This is typically only needed if you managed to damage your LUKS
header, but the container is still mapped, i.e. “luksOpen”ed. It
also helps if you have a mapped container that you forgot or do not
know a passphrase for (e.g. on a long running server.)

WARNING: Things go wrong, do a full backup before trying this!

WARNING: This exposes the master key of the LUKS container. Note
that both ways to recreate a LUKS header with the old master key
described below will write the master key to disk. Unless you are
sure you have securely erased it afterwards, e.g. by writing it to
an encrypted partition, RAM disk or by erasing the filesystem you
wrote it to by a complete overwrite, you should change the master
key afterwards. Changing the master key requires a full data
backup, luksFormat and then restore of the backup.

First, there is a script by Milan that automates the whole
process, except generating a new LUKS header with the old master
key (it prints the command for that though):

The result is actually one line, wrapped here for clarity. The long
hex string is the master key.

– Convert the master key to a binary file representation. You can
do this manually, e.g. with hexedit. You can also use the tool
“xxd” from vim like this:

echo “a1704d9….53d0d09” | xxd -r -p >

– Do a luksFormat to create a new LUKS header.

NOTE: If your header is intact and you just forgot the
passphrase, you can just set a new passphrase, see next
sub-item.

Unmap the device before you do that (luksClose). Then do

cryptsetup luksFormat –master-key-file=

Note that if the container was created with other than the default
settings of the cryptsetup version you are using, you need to give
additional parameters specifying the deviations. If in doubt, try
the script by Milan. It does recover the other parameters as well.

Side note: This is the way the decrypt_derived script gets at the
master key. It just omits the conversion and hashes the master key
string.

– If the header is intact and you just forgot the passphrase, just
set a new passphrase like this:

cryptsetup luksAddKey –master-key-file=

You may want to disable the old one afterwards.

* 6.11 What does the on-disk structure of dm-crypt look like?

There is none. dm-crypt takes a block device and gives encrypted
access to each of its blocks with a key derived from the passphrase
given. If you use a cipher different than the default, you have to
specify that as a parameter to cryptsetup too. If you want to
change the password, you basically have to create a second
encrypted device with the new passphrase and copy your data over.
On the plus side, if you accidentally overwrite any part of a
dm-crypt device, the damage will be limited to the area you
overwrote.

* 6.12 What does the on-disk structure of LUKS look like?

A LUKS partition consists of a header, followed by 8 key-slot
descriptors, followed by 8 key slots, followed by the encrypted
data area.

Header and key-slot descriptors fill the first 592 bytes. The
key-slot size depends on the creation parameters, namely on the
number of anti-forensic stripes, key material offset and master
key size.

With the default parameters, each key-slot is a bit less than
128kiB in size. Due to sector alignment of the key-slot start,
that means the key block 0 is at offset 0x1000-0x20400, key
block 1 at offset 0x21000-0x40400, and key block 7 at offset
0xc1000-0xe0400. The space to the next full sector address is
padded with zeros. Never used key-slots are filled with what the
disk originally contained there, a key-slot removed with
“luksRemoveKey” or “luksKillSlot” gets filled with 0xff. Due to
2MiB default alignment, start of the data area for cryptsetup 1.3
and later is at 2MiB, i.e. at 0x200000. For older versions, it is
at 0x101000, i.e. at 1’052’672 bytes, i.e. at 1MiB + 4096 bytes
from the start of the partition. Incidentally, “luksHeaderBackup”
for a LUKS container created with default parameters dumps exactly
the first 2MiB (or 1’052’672 bytes for headers created with
cryptsetup versions < 1.3) to file and "luksHeaderRestore" restores
them.

For non-default parameters, you have to figure out placement
yourself. "luksDump" helps. See also next item. For the most common
non-default settings, namely aes-xts-plain with 512 bit key, the
offsets are: 1st keyslot 0x1000-0x3f800, 2nd keyslot
0x40000-0x7e000, 3rd keyslot 0x7e000-0xbd800, …, and start of
bulk data at 0x200000.

Note: From cryptsetup 1.3 onwards, alignment is set to 1MB. With
modern Linux partitioning tools that also align to 1MB, this will
result in alignment to 2k sectors and typical Flash/SSD sectors,
which is highly desirable for a number of reasons. Changing the
alignment is not recommended.

That said, with default parameters, the data area starts at
exactly 2MB offset (at 0x101000 for cryptsetup versions before
1.3). The smallest data area you can have is one sector of 512
bytes. Data areas of 0 bytes can be created, but fail on mapping.

While you cannot put a filesystem into something this small, it may
still be used to contain, for example, key. Note that with current
formatting tools, a partition for a container this size will be
3MiB anyways. If you put the LUKS container into a file (via
losetup and a loopback device), the file needs to be 2097664 bytes
in size, i.e. 2MiB + 512B.

There two ways to influence the start of the data area are key-size
and alignment.

For alignment, you can go down to 1 on the parameter. This will
still leave you with a data-area starting at 0x101000, i.e.
1MiB+4096B (default parameters) as alignment will be rounded up to
the next multiple of 8 (i.e. 4096 bytes) If in doubt, do a dry-run
on a larger file and dump the LUKS header to get actual
information.

For key-size, you can use 128 bit (e.g. AES-128 with CBC), 256 bit
(e.g. AES-256 with CBC) or 512 bit (e.g. AES-256 with XTS mode).
You can do 64 bit (e.g. blowfish-64 with CBC), but anything below
128 bit has to be considered insecure today.

Example 1 – AES 128 bit with CBC:

cryptsetup luksFormat -s 128 –align-payload=8

This results in a data offset of 0x81000, i.e. 516KiB or 528384
bytes. Add one 512 byte sector and the smallest LUKS container size
with these parameters is 516KiB + 512B or 528896 bytes.

Example 2 – Blowfish 64 bit with CBC (WARNING: insecure):

cryptsetup luksFormat -c blowfish -s 64 –align-payload=8 /dev/loop0

This results in a data offset of 0x41000, i.e. 260kiB or 266240
bytes, with a minimal LUKS container size of 260kiB + 512B or
266752 bytes.

* 6.14 I think this is overly complicated. Is there an alternative?

Not really. Encryption comes at a price. You can use plain
dm-crypt to simplify things a bit. It does not allow multiple
passphrases, but on the plus side, it has zero on disk description
and if you overwrite some part of a plain dm-crypt partition,
exactly the overwritten parts are lost (rounded up to sector
borders).

* 6.15 Can I clone a LUKS container?

You can, but it breaks security, because the cloned container has
the same header and hence the same master key. You cannot change
the master key on a LUKS container, even if you change the
passphrase(s), the master key stays the same. That means whoever
has access to one of the clones can decrypt them all, completely
bypassing the passphrases.

The right way to do this is to first luksFormat the target
container, then to clone the contents of the source container, with
both containers mapped, i.e. decrypted. You can clone the decrypted
contents of a LUKS container in binary mode, although you may run
into secondary issues with GUIDs in filesystems, partition tables,
RAID-components and the like. These are just the normal problems
binary cloning causes.

Note that if you need to ship (e.g.) cloned LUKS containers with a
default passphrase, that is fine as long as each container was
individually created (and hence has its own master key). In this
case, changing the default passphrase will make it secure again.

7. Interoperability with other Disk Encryption Tools

* 7.1 What is this section about?

Cryptsetup for plain dm-crypt can be used to access a number of
on-disk formats created by tools like loop-aes patched into
losetup. This sometimes works and sometimes does not. This
section collects insights into what works, what does not and where
more information is required.

Additional information may be found in the mailing-list archives,
mentioned at the start of this FAQ document. If you have a
solution working that is not yet documented here and think a wider
audience may be interested, please email the FAQ maintainer.

* 7.2 loop-aes: General observations.

One problem is that there are different versions of losetup around.
loop-aes is a patch for losetup. Possible problems and deviations
from cryptsetup option syntax include:

– Offsets specified in bytes (cryptsetup: 512 byte sectors)

– The need to specify an IV offset

– Encryption mode needs specifying (e.g. “-c twofish-cbc-plain”)

– Key size needs specifying (e.g. “-s 128” for 128 bit keys)

– Passphrase hash algorithm needs specifying

Also note that because plain dm-crypt and loop-aes format does not
have metadata, and while the loopAES extension for cryptsetup tries
autodetection (see command loopaesOpen), it may not always work.
If you still have the old set-up, using a verbosity option (-v)
on mapping with the old tool or having a look into the system logs
after setup could give you the information you need. Below, there
are also some things that worked for somebody.

* 7.3 loop-aes patched into losetup on Debian 5.x, kernel 2.6.32

In this case, the main problem seems to be that this variant of
losetup takes the offset (-o option) in bytes, while cryptsetup
takes it in sectors of 512 bytes each. Example: The losetup command

There is no replacement for –pass-fd, it has to be emulated using
keyfiles, see the cryptsetup man-page.

8. Issues with Specific Versions of cryptsetup

* 8.1 When using the create command for plain dm-crypt with
cryptsetup 1.1.x, the mapping is incompatible and my data is not
accessible anymore!

With cryptsetup 1.1.x, the distro maintainer can define different
default encryption modes for LUKS and plain devices. You can check
these compiled-in defaults using “cryptsetup –help”. Moreover, the
plain device default changed because the old IV mode was
vulnerable to a watermarking attack.

If you are using a plain device and you need a compatible mode, just
specify cipher, key size and hash algorithm explicitly. For
compatibility with cryptsetup 1.0.x defaults, simple use the
following:

cryptsetup create -c aes-cbc-plain -s 256 -h ripemd160

LUKS stores cipher and mode in the metadata on disk, avoiding this
problem.

* 8.2 cryptsetup on SLED 10 has problems…

SLED 10 is missing an essential kernel patch for dm-crypt, which
is broken in its kernel as a result. There may be a very old
version of cryptsetup (1.0.x) provided by SLED, which should also
not be used anymore as well. My advice would be to drop SLED 10.

* 8.3 Gcrypt after 1.5.3 breaks Whirlpool

It is the other way round: In gcrypt 1.5.3 and before Whirlpool is
broken and it was fixed in the next version. If you selected
whirlpool as hash on creation of a LUKS container, it does not work
anymore with the fixed library. Currently, the only work-around is
to decrypt with the old version. This also shows one risk of using
rarely used settings.

9. References and Further Reading

* Purpose of this Section

The purpose of this section is to collect references to all
materials that do not fit the FAQ but are relevant in some fashion.
This can be core topics like the LUKS spec or disk encryption, but
it can also be more tangential, like secure storage management or
cryptography used in LUKS. It should still have relevance to
cryptsetup and its applications.

If you wan to see something added here, send email to the
maintainer (or the cryptsetup mailing list) giving an URL, a
description (1-3 lines preferred) and a section to put it in. You
can also propose new sections.

At this time I would like to limit the references to things that
are available on the web.